The majority of incandescent lamps today use a filament made from tungsten wire which can be of the single or coiled coil design. When initially energized to incandescence, the filament will metallurgically recrystallize, physically sag under gravitational attraction and, in lamps where the filament wire is fine, will become brittle or distorted due to the presence of any carbon in the lamp. Coiled coil filaments, for the most part, have more of a tendency of becoming brittle or distorted than to single coil filaments and fine wire has more of a tendency of becoming brittle or distorted than does heavy wire.
Embrittlement and coil distortion in tungsten filaments have never been completely eliminated. However, they can be significantly reduced by utilizing a controlled heating process at the time of initial light-up. Flashing is one particular process known for doing this and is now in common use. Briefly, flashing is a method of stabilizing a filament. It is usually done after the coil is mounted in the lamp and can be performed either before or after tip-off. Since the filament as received is not brittle, it does not require hand mounting and, therefore, can be mounted inexpensively via high speed automatic equipment.
Unfortunately, a fine wire filament in an incandescent lamp will continue to deteriorate and become more brittle during subsequent lamp operation in spite of flashing. This is generally attributed to the formation of tungsten carbide, a more brittle material (due to excess carbon), at the surface of the filament. The formation of tungsten carbide leads to slippage at the grain boundaries at the surface of the filament. The embrittlement problem is significant in fine wire filaments (less than 0.005 inch diameter) since there is less material in the cross-section of the wire to overcome defects or weaknesses in the surface of the filament wire.
It is believed, therefore, that a tungsten-halogen lamp that provides for means for substantially reducing filament embrittlement within a lamp would constitute an advancement in the art.